An on-line roll grinding apparatus grinds a work roll (may hereinafter be referred to as a roll) during the operation of a rolling mill to eliminate irregularities on the surface of the work roll occurring upon rolling, thereby increasing the volume of production (roll units) achievable by rolling with a single roll. Such an on-line roll grinding apparatus has so far been used, for example, in a finishing mill in the continuous hot rolling process (HOT).
That is, when the work roll of a plate rolling mill rolls a steel plate, only portions of the work roll in contact with the steel plate wear, and wear steps appear between the worn-out portions and non-rolling portions out of contact with the steel plate. If the roll with such wear steps is used to roll a steel plate having a width equal to or larger than the widths of the wears, the problem of deteriorating the accuracy of the plate thickness or the flatness has arisen.
This has necessitated restrictions on a rolling schedule, such as the rolling of plates in the order of decreasing width. However, the installation of an on-line roll grinding apparatus enables the wear steps, which increase with the progress of rolling, to be sequentially ground and removed. Thus, the above restrictions can be abolished to increase the roll units.
With the on-line roll grinding apparatus, in order to avoid damage to a rotating grinding wheel (may hereinafter be referred to as a grinding wheel) or failure of the grinding apparatus, wear steps of the non-rolling portions have to be ground and removed within a limited period of time which is other than the instant of biting, into the rolling mill, of a material to be rolled, and the instant of release of the rolled material from the rolling mill.
In order to grind and remove the wear steps efficiently, therefore, it is necessary to use a grinding wheel having a high grinding ability, and ensure as long an actual grinding time as possible.
In order that the grinding wheel is not damaged upon contact of the roll and the grinding wheel due to vibrations caused by impact at the time of biting, into the rolling mill, of the material to be rolled and release of the material from the rolling mill, the grinding wheel, when not in a grinding operation, is on standby 10 mm to 20 mm away from the roll surface.
If the timing is right for start of grinding, the grinding wheel begins to move forward, and touches the roll. Then, when a grinding pressing load which has been set beforehand is reached, grinding is started. To ensure a long actual grinding time, a grinding preparation time from the start of forward movement of the grinding wheel until the above set grinding pressing load is reached needs to be shortened. That is, the velocity of the forward movement (i.e., forward velocity) of the grinding wheel needs to be rendered high.
The grinding ability of the grinding wheel varies with its pressing load on the roll. During grinding, therefore, it is necessary to detect the pressing load by a built-in load cell, and control the pressing amount of the grinding wheel, thereby controlling the pressing load. The structure of the grinding apparatus, and the basic method of its control are disclosed, for example, in Patent Document 1.
This method, basically, comprises grinding under a certain pressing load to a certain depth in order to maintain the profile of the roll. If the profile is to be changed intentionally, the pressing load is varied according to the site of the roll to control the grinding depth.
However, this conventional method has posed the problem that if the grinding wheel is moved forward at a high velocity at the start of grinding, the pressing load increases rapidly to greatly exceed the set value of the grinding pressing load, thereby causing excessive grinding or, in some cases, damaging the grinding wheel.
Particularly, in the conventional on-line grinding in the rolling mill for the continuous hot rolling process, grinding was performed during biting of the material to be rolled, which lasted for a relatively long time of 60 seconds to 120 seconds. Because of this ample grinding time, the necessity for shortening the grinding preparation time was minimal. Thus, priority was placed on preventing excessive grinding or damage to the grinding wheel, and the adopted method was to move the grinding wheel forward from the standby position at a low velocity and bring the grinding wheel into touch with the roll.
In rolling a thick plate with a plate thickness of 6 mm or more, however, intermittent rolling by one or two rolling mills is performed. Thus, the rolling time, during which the material to be rolled is bitten and rolled, is of the order of 10 seconds at the longest. With this period of time, roll grinding cannot be completed at a stroke. The timing with which grinding can take place is at the time of a turn when the direction of rolling of the steel plate is changed, or at idle.
The state “at idle” refers to a state in which after rolling of a preceding material to be rolled has been completed, the roll is idling while waiting for biting of a succeeding material to be rolled, the state lasting for a short time of about 20 seconds.
Under these circumstances, it was necessary to actualize an on-line roll grinding method for ensuring as long a roll grinding time as possible, and decreasing an overshoot, by which the pressing load of the grinding wheel on the work roll exceeds the set grinding pressing load, thereby preventing excessive grinding or damage to the grinding wheel.
Patent Document 1: Japanese Patent Application Laid-Open No. 1996-30941.1